The present invention relates to air bag devices having an air bag and a cover disposed thereon.
Air bag devices are known conventionally, which are arranged, for example, at a boss of a steering wheel of a motor vehicle to absorb impact on a driver upon collision. The air bag device comprises an inflator for jetting out gas, an air bag arranged in fold, a cover disposed on the air bag, and a base plate having the inflator, the air bag, and the cover mounted thereon. The air bag device is constructed such that when detecting collision of the motor vehicle, i.e. its sudden deceleration over a predetermined value, the inflator jets out gas instantaneously to distend the air bag. Its pressure serves to break the cover along a predetermined tear line for door-like extension, from which the air bag protrudes to develop in front of the driver.
The cover includes a covering portion for covering the folded air bag and a mounting portion that protrudes downwardly from the covering portion, having substantially a box-like form with an opening on the lower side. Likewise, the base plate includes a base portion having the inflator and the air bag mounted thereon and a support portion that protrudes downwardly from the base portion, having substantially a box-like form with an opening on the lower side. With the mounting portion engaged with the outer periphery of the support portion, the mounting portion, the support portion, etc. are fixed to each other by a number of rivets, obtaining the cover fixed to the base plate.
However, the structure using a number of rivets for fixing the cover is difficult to reduce a manufacturing cost due to the increased number of component parts and complicated assembling process.
JP-A 6-298031 proposes another structure for fixing the cover, wherein the mounting portion of the cover and the support portion of the base plate are formed cylindrically. With the mounting portion engaged with the outer periphery of the support portion, a fastening band is tightly wound on the outer periphery of the mounting portion, obtaining the mounting portion held between the support portion and the band.
However, the structure using the fastening band is difficult to control the fastening strength due to the need of applying an appropriate fastening force for assembling the cover to the base plate. Further, due to the fact that no means are provided to position the fastening band, sure fixing of the cover needs to fasten the band in holding it in an accurate position, resulting in complicated fastening work. Furthermore, the use of the fastening band is not available except the cover and the base plate having a rounded section such as a circle, resulting in limited form of the cover and the base plate.
JP-A 6-144139 proposes still another structure for fixing the cover, wherein with a pair of substantially C-shaped plates disposed on the outer periphery of the mounting portion of the cover engaged with the support portion of the base plate, the support portion, the mounting portion, and the plates are integrally mounted by a number of rivets. Each plate is formed with claws at the lower end, which are engaged with the support portion of the base plate and the mounting portion of the cover for positioning.
However, the structure using the claws has a problem that the claws, arranged for positioning, cannot support the mounting portion against the distending pressure of the air bag, requiring a number of rivets for fixing the air bag, resulting in a difficult reduction in a manufacturing cost due to the increased number of component parts and complicated assembling process.
As described above, the structure using the fastening band needs not only to apply an appropriate fastening force for assembling the cover to the base plate, but to fasten the band in holding it in an accurate position, resulting in complicated fastening work and limited form of the cover and the base plate. On the other hand, the structure using the claws has a problem that the claws, arranged for positioning, cannot support the mounting portion against the distending pressure of the air bag, requiring a number of rivets for fixing the air bag, resulting in a difficult reduction in a manufacturing cost due to the increased number of component parts and complicated assembling process.
It is, therefore, an object of the present invention to provide air bag devices that enable easy and secure fixing of the cover to the base plate.